Final answer:
A 50% decrease in efferent arteriolar resistance would cause the greatest decrease in GFR in a person with otherwise normal kidneys.
Step-by-step explanation:
The greatest decrease in GFR in a person with otherwise normal kidneys would be caused by a 50% decrease in efferent arteriolar resistance (option e). When the resistance in the efferent arteriole decreases, more blood will exit the glomerulus, resulting in a decrease in filtration and a decrease in GFR. This is because the efferent arteriole plays a crucial role in regulating the pressure within the glomerulus.
The other options would not cause as significant of a decrease in GFR. For example, a decrease in renal arterial pressure (option a) would affect the overall blood flow to the kidneys, but it would not directly impact GFR. Similarly, changes in glomerular capillary filtration coefficient (option b) or proximal tubular sodium reabsorption (option c) would have a lesser effect on GFR.
Therefore, the 50% decrease in efferent arteriolar resistance would cause the greatest decrease in GFR in a person with otherwise normal kidneys.